This application is based on application No. H11-355676 filed in Japan on Dec. 15, 1999, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image-sensing apparatus, such as a digital camera, and particularly to an apparatus in which the optical path is split by a half mirror so that light from a taking lens is partially directed to an optical viewfinder.
2. Description of the Prior Art
Generally, a digital camera is equipped with a display device such as a liquid crystal display device, and this display device is used both to display a playback of an image shot and stored previously and to display an image just being shot. This enables the user to determine picture composition and confirm focus condition while viewing a xe2x80x9clive viewxe2x80x9d, i.e. the image just being shot, that is displayed on the display device. Thus, the display device functions as a video viewfinder.
Some digital cameras are equipped with, in addition to a display device, an optical viewfinder. Digital cameras equipped with an optical viewfinder divide into a separate type, in which light is introduced to the viewfinder without being passed through a taking lens, and a single-lens-reflex (SLR) type, in which the light having passed through a taking lens is reflected so as to be introduced to the viewfinder. Whereas the separate type suffers parallax between the viewfinder and the taking lens, the SLR type has the advantage of intrinsically being free of parallax.
The SLR type requires a prism to be disposed in the optical path from the taking lens to an image-sensing device so as to reflect a part and transmit the other part of the light having passed through the taking lens, and requires also a pentaprism for directing the reflected light to the eye of the user in such a way as to permit the user to observe an erect image. In this arrangement, the optical viewfinder and the video viewfinder can be used at the same time. However, only part of the light having passed through the taking lens reaches the image-sensing device all the time, and therefore light is used inefficiently in shooting.
As is well known, in an SLR-type camera that shoots by exposure of silver-halide film, it has been customary to dispose a total-reflection mirror obliquely in the optical path from the taking lens to the film in such a way that the mirror can be rotated out of the optical path. Until immediately before shooting, i.e. exposure of film, the mirror is kept in the optical path so that the incoming light is reflected toward the pentaprism. Only at the moment of shooting is the mirror retracted out of the optical path so that the incoming light is directed to the film, and, immediately after shooting, the mirror is brought back in the optical path. This type of mirror is called a quick-return mirror.
Digital cameras equipped with such a quick-return mirror are already in practical use, producing bright images despite using an optical viewfinder. However, in this arrangement, while the total-reflection mirror is placed in the optical path to permit the use of the optical viewfinder, no light reaches the image-sensing device, and thus the display device cannot be used as a viewfinder.
One way to overcome this inconvenience is to equip digital cameras with a half mirror that acts as a quick-return mirror. This permits, whether the half mirror is placed in or retracted out of the optical path, the incoming light to reach the image-sensing device, and thus makes both shooting and display possible at any time. While the half mirror is placed in the optical path, the optical viewfinder can be used as well. On the other hand, when an image to be recorded is shot, the half mirror is retracted out of the optical path so that all the light from the taking lens is used for shooting.
As a result of this improvement, digital cameras have come to offer, in addition to their own features, operability close to that of cameras that shoot by exposure of silver-halide film (hereinafter called silver-halide cameras). However, whereas a silver-halide camera provides varying sensitivity according to the film loaded therein, a digital camera has an image-sensing device fixed therein, and therefore intrinsically provides constant sensitivity. In this respect, digital cameras greatly differ from silver-halide cameras, offering less flexibility in terms of settings for exposure control such as the aperture value and the shutter speed.
One way to overcome this inconvenience is to vary effective sensitivity by varying the gain of the output signal of the image-sensing device. However, when extremely intense light is incident on the image-sensing device, the output thereof becomes saturated, in which case, even if the gain is lowered, it is impossible to reproduce highlight areas properly. One way to avoid this is to insert an ND filter in the optical path from the taking lens to the image-sensing device so as to reduce the very amount of light incident on the image-sensing device and thereby lower apparent sensitivity. The user, by using these sensitivity switching functions, can control exposure to a certain extent.
In fact, in some digital cameras, the brightness of the subject is detected and, when the subject is found to be too bright to be shot with proper exposure, an ND filter is automatically inserted in the optical path.
However, equipping a camera with an ND filter and a mechanism for its insertion makes the construction of the camera as a whole unduly large. Moreover, disposing an ND filter in front of a prism or half mirror, i.e. an optical member that splits the optical path, makes the image observed through an optical viewfinder unduly dim.
An object of the present invention is to provide an image-sensing apparatus that has an optical member retractable out of the optical path and that permits flexible exposure control despite being compact with minimum degradation of image quality.
Another object of the present invention is to provide, in an image-sensing apparatus having an optical member retractable out of the optical path, a method for flexibly controlling the exposure of an image-sensing device and a method for driving such an optical member in a manner suitable for exposure control.
To achieve the above objects, according to one aspect of the present invention, an image-sensing apparatus is provided with: a taking lens; an image-sensing device for shooting an image by receiving light from the taking lens; an optical member that can be moved between an intercepting position in which the optical member crosses the optical path from the taking lens to the image-sensing device and a retracted position in which the optical member lies off the optical path; a sensor for measuring the brightness of a subject; an operation member operated by a user; a driver for positioning the optical member in the intercepting or retracted position according to an instruction fed in through the operation member; and a controller for adjusting the amount of exposure of the image-sensing device according to the output of the sensor and the position of the optical member.
In this image-sensing apparatus, according to an instruction fed in by the user operating the operation member, the optical member is positioned in the intercepting or retracted position so that, according to the position of the optical member, the amount of light incident on the image-sensing device is varied and thereby the apparent sensitivity of the image-sensing device is switched. While the controller controls the amount of exposure of the image-sensing device, the controller does so with consideration given not only to the brightness of the subject but also to the position of the optical member, and therefore the instruction from the user is reflected in the amount of exposure of the image-sensing device thus adjusted. This makes flexible exposure control possible. The controller may vary either or both of the aperture value and the electronic shutter speed. As the optical member, a half mirror is used, for example.
The image-sensing apparatus may be further provided with an adjuster for adjusting the intensity of a signal representing an image shot according to the position of the optical member so that the controller adjusts the amount of exposure of the image-sensing device according to the intensity of the signal adjusted by the adjuster.
In this arrangement, the adjuster, by making the intensity of the signal representing the image shot higher or lower, can vary the effective sensitivity of the image-sensing device. Since the adjuster adjusts the intensity of the signal according to the position of the optical member, both the switching of the apparent sensitivity of the image-sensing device and the varying of the effective sensitivity thereof are performed according to the instruction from the user. Moreover, the controller adjusts the amount of exposure of the image-sensing device with consideration given also to the signal intensity. This makes more flexible exposure control possible.
Here, the adjuster may be so configured as to make the intensity of the signal higher when the optical member is in the retracted position than when it is in the intercepting position. This makes it possible to set the sensitivity of the image-sensing device to be higher in the retracted position and lower in the intercepting position than when the signal intensity is kept constant. That is, it is possible to make the range of sensitivity of the image-sensing device wider than when simply the position of the optical member is changed.
Alternatively, the adjuster may be so configured as to make the intensity of the signal lower when the optical member is in the retracted position than when it is in the intercepting position. This makes it possible to set the sensitivity of the image-sensing device to be lower in the retracted position and higher in the intercepting position than when the signal intensity is kept constant. Thus, it is possible to obtain medium sensitivity between the maximum and minimum sensitivity obtained when simply the position of the optical member is changed.
The image-sensing apparatus may be further provided with an optical viewfinder for offering a visible image by receiving light reflected from the optical member when it is in the intercepting position. This makes it possible to offer an optical image of the subject when the optical member is in the intercepting position.
According to another aspect of the present invention, in an image-sensing apparatus having a taking lens, an image-sensing device for shooting an image by receiving light from the taking lens, and an optical member that can be moved between an intercepting position in which the optical member crosses the optical path from the taking lens to the image-sensing device and a retracted position in which the optical member lies off the optical path, a method for controlling the amount of exposure of the image-sensing device includes: a step of measuring the brightness of a subject; a step of receiving an instruction from outside; a step of positioning the optical member in the intercepting or retracted position according to the instruction received from outside; and a step of setting the amount of exposure of the image-sensing device according to the measured brightness of the subject and the position of the optical member.
This method permits the user to feed in an instruction to move the optical member to the intercepting or retracted position. In addition, since the amount of exposure of the image-sensing device is determined according to both the brightness of the subject and the position of the optical member, it is possible to reflect the user""s intention in the adjustment of exposure.
According to still another aspect of the present invention, an image-sensing apparatus is provided with: a taking lens; an image-sensing device for shooting an image by receiving light from the taking lens; an optical member that can be moved between an intercepting position in which the optical member crosses the optical path from the taking lens to the image-sensing device and a retracted position in which the optical member lies off the optical path; a sensor for measuring the brightness of a subject; and a driver for positioning the optical member in the intercepting position when the brightness of the subject measured by the sensor is equal to or higher than a predetermined level and for positioning the optical member in the retracted position when the brightness of the subject measured by the sensor is lower than the predetermined level.
In this image-sensing apparatus, the amount of light incident on the image-sensing device is automatically switched according to the brightness of the subject. By keeping the optical member in the retracted position, it is possible to use all the light from the subject for shooting. This makes it possible to control exposure flexibly even when the subject is dim. For example, it is possible to stop down the aperture stop further. On the other hand, by positioning the optical member in the intercepting position, it is possible to reduce the amount of light reaching the image-sensing device. This makes it possible to control exposure flexibly even when the subject is very bright. For example, it is possible to open the aperture closer to the fully open aperture. As the optical member, a half mirror is used, for example.
The image-sensing apparatus may be further provided with a display for notifying the user of a change in the position of the optical member. This permits the user to confirm in which position the optical member is when shooting is performed and thereby prevent faulty adjustment of the amount of exposure. The display may be used also to display an image shot.
According to a further aspect of the present invention, in an image-sensing apparatus having a taking lens, an image-sensing device for shooting an image by receiving light from the taking lens, and an optical member that can be moved between an intercepting position in which the optical member crosses the optical path from the taking lens to the image-sensing device and a retracted position in which the optical member lies off the optical path, a method for driving the optical member includes: a step of measuring the brightness of a subject; a step of positioning the optical member in the intercepting position when the measured brightness of the subject is equal to or higher than a predetermined level; and a step of positioning the optical member in the retracted position when the measured brightness of the subject is lower than the predetermined level.
By this method, when the subject is dim, all the light from the taking lens is directed to the image-sensing device, and, when the subject is bright, only part of the light from the taking lens is directed to the image-sensing device. This makes it possible not only to shoot a dim subject with proper brightness but also to shoot a very bright subject with proper brightness. In addition, since the position of the optical member is changed automatically according to the brightness of the subject relative to the predetermined level, the user does not need to perform any extra operation for that.